Diastereoselectivity in Chiral Osmium Complexes of a Bidentate Bisphosphine Monoxide Ligand

The chelation of [(η 6 -Cy)OsL] fragments (Cy = cymene; L = monodentate ligand) by bisphosphine monoxide ligands generates chiral-at-metal complexes. If the bisphosphine monoxide backbone contains a chiral center, diastereomeric products are formed. Osmium complexes of this type prepared using rac -Ph 2 PCH(Me)P(O)Ph 2 epimerize to form a preferred isomer in which the methyl group is distal to L. This has been observed for the analogous ruthenium complexes, but the interconversion of diastereomers occurs at a greatly reduced rate. The slower interconversion allows spectroscopic characterization of the thermodynamically less favored isomers, as the corresponding ruthenium species are too short-lived for such investigations.